Certain wireless communication systems, such as for example wireless local area networks (WLAN) or universal mobile telecommunication systems (UMTS) are discussed to operate in frequency bands, which are also used by radar systems. This co-existence of a wireless communication system with a radar system leads to the fact that the frequency bands from both systems overlap at least partly. For example WLAN systems, like HIPERLAN/2 or IEEE802.11a, are planned to operate in the frequency range from 5150 MHz to 5350 MHz and 5470 MHz to 5725 MHz, while radar systems are using the frequency range from 5250 MHz to 5850 MHz.
Currently, when a wireless communication system in the 5 GHz range is planned to co-exist with a radar system, it is a regulation that the radar system is the primary user. Hence, it is mandatory to avoid that the wireless communication system interferes with the radar system. Therefore, to avoid a possible conflict with the radar system, the wireless communication system has to vacate the frequencies, currently in use by the radar system.
A typical radar system transmits signals in pulses with a pulse length from around 0.05 to 100 μs and a transmission power in the range up to 60 dBW. With this knowledge, a receiver in the wireless communication system is able to distinguish the radar signals from other signals transmitted in the wireless communication system. If e.g. for an observed frequency, the signal strength from a received signal is in such a time period from around 0.05 to 100 μs above a certain value, it is assumed that this frequency is actually used by a radar system. Then, the wireless communication system has to remove this frequency from the list of allowed transmission frequencies or has to change the currently used frequency to avoid any harmful interference to the radar system.
If, in a receiver, e.g. the signal strength is measured after an analog-to-digital converter stage in the digital part, the indication which frequency is currently used from a radar system is ambiguous. This measured signal strength can be above a threshold either due to a received radar signal, which is transmitted on the currently observed frequency or from a radar signal, which is transmitted on some other frequencies, but converted to the observed frequency. Such other frequencies can be converted to the currently observed frequency for example by means of image frequency conversion or aliasing effects. The image frequency conversion results from the arrangement of the analog part in the receiver as a heterodyne receiver with a limited image rejection. The aliasing effects results from the analog-to-digital-converter stage in the receiver, which converts the received signal at a limited sampling rate. Hence, if the detection is arranged in the digital part of the receiver, more frequencies are wrongly determined as possibly in use by a co-channel radar system. Then, more frequencies are determined as to be vacated as really needed and thus the number of usable frequencies for transmission in the wireless communication system is limited more than required.